Process for producing aging-resistant rubber material

ABSTRACT

An aging-resistant rubber material with an improved heat resistance without deteriorating the moldability even if a rubber layer is formed from an aqueous latex of rubber and without any cost increase, even if a surface tack prevention treatment by an antitack agent is carried out, can be produced by applying a coating liquid containing an amine-based antioxidant in solution or dispersion to the surface of a cross-linked rubber material, followed by heating to a temperature of 80° C. or higher, thereby diffusing the amine-based antioxidant into the rubber material. When a coating solution containing an antitack agent together with the amine-based antioxidant in solution or dispersion is used, the surface tack prevention treatment of the cross-linked material as a product can be made at the same time.

TECHNICAL FIELD

The present invention relates to a process for producing anaging-resistant rubber material, and more particularly to a process forproducing an aging-resistant rubber material with improved heatresistance while maintaining the moldability.

BACKGROUND ART

Improvement of the heat resistance of rubber material has been so farmade by addition of an antioxidant thereto. Heat resistance and agingresistance can be very effectively improved by the addition of theantioxidant, but further improvement of heat resistance is still now arequirement for prolonging the product life.

However, it is the accepted knowledge that there is an optimal amount ofthe antioxidant to be added, and even a simple increase in the amount ofantioxidant to be added can never contribute to a desired functionalimprovement, corresponding to such an increment of the addedantioxidant. Addition of a larger amount of the antioxidant than theoptimal amount will inhibit the cross-linking reaction and also willlower the physical properties of the resulting rubber material. From theviewpoint of productivity, high speed cross-linking of rubber isrequired, and cross-linking retardation (cross-linking inhibition) hasan adverse effect on product cost. Thus, it has been desired to attainhigh speed cross-linking and also to improve the product life, withoutaddition of a large amount of an antioxidant to a rubber layer.

Rubber materials are often used upon compositing with metallicmaterials, where, for example, an aqueous latex of rubber is applied toa metallic sheet, followed by cross-linking to conduct compositing. Inthat case, an antioxidant must be added to an aqueous latex of rubber tomake the resulting rubber layer contain the antioxidant, but by theinitial addition of the antioxidant the latex will, in some cases, loosethe dispersion stability, resulting in gellation and molding failure ofrubber materials, whereas without the addition of the antioxidant noimprovement of the heat resistance of the rubber materials can beattained. This is a problem.

Generally, the rubber materials have some tackiness due to a flexibilityin the nature, so extraneous matters such as dusts, etc. are liable toattach to the tacky surface, creating disagreeable touch and mutualadhesion of rubber products themselves, and thus surface coating with anantitack agent is often carried out to improve the surface tackiness.However, such a surface tack prevention treatment leads to increasingcost of rubber products, so their application and use are oftenrestricted.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a process for producingan aging-resistant rubber material capable of improving the heatresistance while maintaining the moldability even in case of forming arubber layer, using an aqueous latex of rubber, and having nothing to dowith increasing cost even in case of conducting a surface tackprevention treatment with an antitack agent.

The object of the present invention can be attained by applying asolution or a dispersion of an amine-based antioxidant as a coatingliquid to the surface of cross-linked rubber material, followed byheating to 80° C. or higher, thereby diffusing the amine-basedantioxidant into the rubber material to produce an aging-resistantrubber material. When a coating liquid containing an antitack agenttogether with the amine-based antioxidant in solution or dispersion isused, the surface tack prevention treatment of cross-linked rubbermaterials as products can be carried out at the same time.

Cross-linked rubber materials for use in the present invention includesrubber materials such as acrylic rubber, NBR, hydrogenated NBR, SBR,fluoroelastomer, etc. as subjected to cross-linking molding beforehand.Rubber materials containing a normal quantity or less quantity of theamine-based antioxidant beforehand may be used. In the presentinvention, the amine-based antioxidant is further diffused into therubber materials from the surface, and thus it is not necessary that therubber materials contains a sufficient quantity or an excess quantity ofthe amine-based antioxidant at the time of rubber cross-linking. Thus,cross-linking inhibition or lowering of physical properties by theantioxidant can be prevented thereby.

In the present invention, no antioxidant is contained at all in therubber material at the time of cross-linking, or, if contained, asubstantially small quantity of the antioxidant will do at the time ofcross-linking, so not only cross-linking inhibition or lowering ofphysical properties by the antioxidant can be prevented, but also highspeed cross-linking can be easily attained. Furthermore, the antioxidantas coated on the surface will diffuse into the rubber materials from thesurface, so it is expectable that there is a much larger quantity of theantioxidant in the region near the surface, and drastic improvement ofheat resistance can be attained. Generally, heat aging occurs due to theaction of oxygen in the air and thus proceeds from the rubber surfacelayer inwardly. In the present invention, the antioxidant is unevenlydistributed much. more in the surface layer region, and the improvementof heat resistance is more prominent, as compared with that in the caseof simple uniform compounding.

Particularly in the case that the rubber layer is a peroxidecross-linking type or is to be formed from an aqueous emulsion, directaddition of the antioxidant to rubber is restricted. That is, in thecase of peroxide cross-linking type, most of the antioxidants act as asuppressing agent against the organic peroxide cross-linking reaction,so neither added organic peroxide nor antioxidant can performsatisfactory functions corresponding to the quantity as added thereto.In the case of the aqueous emulsion type, gellation will be easy to takeplace by addition of the antioxidant thereto, as already mentionedbefore. In the present invention, on the other hand, the antioxidant ismade to diffuse into the cross-linked rubber layer by coating, wherebysuch inconveniences as cross-linking inhibition or gellation of thecoating liquid can be substantially eliminated.

Such effective diffusion of the amine-based antioxidant into thecross-linked rubber materials is carried out by applying a coatingliquid containing the amine-based antioxidant in solution or dispersionto the surface of the cross-linked rubber materials, followed by heatingto 80° C. or higher.

The amine-based antioxidant for use in the present invention includes,for example, 2,2,4-trimethyl-1,2-dihydroquinoline polymer,6-ethoxy-1,2-dihydro-2,2,4-trimethylquinoline, phenyl-1-naphthylamine,lower alkylated diphenylamine, octylated diphenylamine,4,4′-bis(α,α-dimethylbenzyl)-diphenylamine,p-(p-toluenesulfonylamido)diphenylamine,N,N′-di(2-naphthyl)-p-phenylenediamine,N,N′-diphenyl-p-phenylenediamine,N-phenyl-N′-isopropyl-p-phenylenediamine,N-phenyl-N′-(1,3-dimethyl-butyl)-p-phenylenediamine,N-phenyl-N′-(3-methacryloyloxy-2-hydroxy-propyl)-p-phenylenediamine,etc. Other antioxidants than the amine-based ones, for example, phenolbased one or sulfur based one have no appreciable effect upon the heatresistance improvement.

These antioxidants are used as post-additive antioxidants, separate fromthe initial additive ones, in a proportion of about 0.1 to about 10 wt.%, preferably about 0.2 to about 5 wt. % on the basis of the weight ofthe rubber layer. Below about 0.1 wt. %, the desired effect of thepresent invention upon the heat resistance improvement can be hardlyobtained, whereas a proportion above about 10 wt. % cannot be costwiserecommended.

An antitack agent such as paraffinic wax, graphite, etc. can be usedtogether with the amine-based antioxidant to improve the heat resistanceand prevent the surface tackiness at the same time. It is desired to usethe antitack agent in a ratio by weight of about 0.5 to about 300,preferably about 0.5 to about 200, to the amine-based antioxidant.

The amine-based antioxidant or the antitack agent together are used asdissolved or dispersed in at least one of ketones such as acetone,methyl ethyl ketone, methyl isobutyl ketone, etc., aromatic hydrocarbonssuch as toluene, xylene, etc. and alcohols such as methanol, ethanol,isopropanol, etc. as a coating liquid. In the case that these organicsolvents are not available or their use is to be avoided, they are usedas an aqueous dispersion.

The coating liquid containing the amine-based antioxidant or theantitack agent together as dissolved or dispersed (concentration: about1 to about 80 wt. %, preferably about 2 to about 50 wt. %) can beapplied to the surface of cross-linked rubber materials by an ordinarymethod, such as spray coating, dipping, roll coating, flow coating, etc.

After the coating, fixation of the antioxidant (and the antitack agent)to the surface and diffusion thereof into the rubber layer can becarried out by heat-drying. Temperature and duration of heat treatmentdepend on the heat resistance of the rubber layer to be treated and thespecies of solvents capable of dissolving the surface layer, but areusually 80° to 300° C. for about 10 seconds to about 20 minutes,preferably about 20 seconds to about 10 minutes. Such heat treatment canbe carried out divisionally in a plurality of runs.

To keep a concentration of amine-based antioxidant diffused into therubber layer, for example, at minimum 0.5 phr in the every thicknessdirection, the rubber layer having a thickness of 5 to 300 μm,preferably 5 to 150 μm, must be formed, because concentration of theantioxidant diffused into the rubber layer will be continuouslydecreased from the surface layer inwardly, and will be about 1 phr atthe thickness level of about 300 μm.

In the present invention, diffusion of the antioxidant (and the antitackagent) into the rubber layer can be effectively carried out in the caseof a rubber layer of metal-rubber laminate materials to improve the heatresistance (and to prevent tackiness) of the rubber layer laminated withmetallic sheets.

BEST MODES FOR CARRYING OUT THE INVENTION

The present invention will be described below, referring to Examples.

Example 1

Parts by weight Acrylic rubber (PA-402, a product of Unimatec Co.) 100HAF carbon black 55 Stearic acid 1 4,4′-bis(α,α-dimethylbenzyl)diphenylamine 2 (Antioxidant CD, a product of Ouchi-Shinko Kagaku Co.)Sulfur 0.3 Potassium stearate 3 Sodium stearate 0.25

The foregoing components were kneaded through a 10 inch open roll mill.The resulting kneaded mixture was subjected to press cross-linking at180° C. for 8 minutes and then to oven cross-linking at 175° C. for 4hours, whereby an acrylic rubber sheet having a thickness of 2 mm wasobtained.

The resulting acrylic rubber sheet was dipped into an antioxidantsolution A (6 wt. % of antioxidant CD in toluene) for 5 seconds, thentaken out therefrom, and air dried, followed by heat treatment at 160°C. for 180 seconds. The amount of the antioxidant diffused into therubber sheet by the dipping operation was determined by gravimetricanalysis, and found to be about 0.5 parts by weight. Total amount ofAntioxidant CD ultimately taken into the acrylic rubber was 2.5 parts byweight (on the charge basis).

Then, the following test was conducted:

-   -   Heat resistance: Heated air aging test was carried out at        175° C. for 70 hours to determine pre-test rubber hardness and        post-test change in rubber hardness

Comparative Example 1

In Example 1, the dipping into the antioxidant solution A was notcarried out.

Comparative Example 2

In Example 1, the amount of antioxidant CD in the acrylic rubber kneadedmixture was changed to 2.5 parts by weight, and the dipping into theantioxidant solution A was not carried out.

Results of determination and evaluation in Example 1 and ComparativeExamples 1˜2 are shown in the following Table 1.

TABLE 1 Heat resistance Ex. 1 Comp. Ex. 1 Comp. Ex. 2 Before heat agingJIS A hardness 67 67 65 After heat aging Change in hardness (points) +6+9 +9

Example 2

Parts by weight NBR (N237, a product of JSR) 100 SRF carbon black 60N-phenyl-N′-(1,3-dimethylbutyl)-p-phenylene- 2 diamine (Antioxidant 6C,a product of Ouchi-Shinko Kagaku Co.) Organic peroxide (Percumyl D) 4Solvent (toluene) 360

The foregoing components were mixed together, and the resulting NBRsolution was subjected to lamination to a thickness of 80 μm on a SPCCsteel sheet (thickness: 200 μm) through a phenol resin-based adhesive(Thixon-7, a product of Morton International Co.), followed by presscross-linking at 220° C. for 30 seconds to obtain a metal-NBR laminatematerial.

The resulting metal-NBR laminate material was dipped into an antioxidantsolution B (6 wt. % solution in isopropanol of 2 parts by weight ofAntioxidant 6C and 4 parts by weight of an antitack agent, G79R, aproduct of Resino color Co.) for 5 seconds, taken out therefrom, and airdried, followed by heat treatment at 180° C. for 90 seconds. The amountof Antioxidant 6C diffused into the rubber layer of the metal-rubberlaminate material by the dipping operation was determined by gravimetricanalysis, and found to be about 2.0 parts by weight. Total amount ofAntioxidant 6C ultimately taken into NBR was 4.0 parts by weight (chargebasis).

The following tests were conducted:

-   -   Heat resistance: Heated air aging test was carried out at        120° C. for 70 hours to determine pre-test pencil hardness and        post-test pencil hardness    -   Tackiness: A SUS304 steel sheet was used as a cover plate at the        time of press cross-linking to observe the tackiness between the        NBR and the cover plate    -    As to the laminate materials treated with an antitack agent,        two sheets of the laminate materials were laid one upon another        on the NBR sides, followed by pressing under a pressing force of        0.5 MPa to observe the tackiness there between

Comparative Example 3

In Example 2, dipping of the metal-rubber laminate material was carriedout not in the antioxidant solution B, but in an antitack agent solution(4 wt. % of antitack agent G79R solution in isopropanol).

Comparative Example 4

In Comparative Example 3, the amount of Antioxidant 6C in the NBRkneading product was changed to 4 parts by weight.

Comparative Example 5

In Comparative Example 4, the press cross-linking time at 220° C. waschanged from 30 seconds to 60 seconds.

Comparative Example 6

In Example 2, the dipping into the antioxidant solution B was notcarried out.

Example 3

Parts by weight NBR latex (Nipol 1571, a product of Nippon Zeon 250 Co.,solid concentration: 40 wt. %) SRF carbon black 50 Zine white No. 3 10Organic peroxide (Perhexa 25B) 3

An aqueous mixture consisting of the foregoing components was applied toan SPCC steel sheet through a phenol resin-based adhesive (Thixon P-7)and dried to obtain a laminate with a thickness of 120 μm. Then, presscross-linking was carried out at 220° C. for 30 seconds to obtain ametal-NBR laminate material.

The resulting metal-NBR laminate material was dipped into an antioxidantsolution B for 5 seconds, taken out therefrom and air dried, followed byheat treatment at 180° C. for 90 seconds. The amount of Antioxidant 6Cdiffused into the rubber layer of the metal-NBR laminate material by theforegoing operation was determined by gravimetric analysis and found tobe about 2.0 parts by weight (in term of charge basis).

Comparative Example 7

In Example 3, 2 parts by weight (in terms of charge basis) ofAntioxidant 6C was added to the aqueous mixture, and it was tried toobtain a laminate material, using the aqueous mixture, but it was foundthat the polymer was coagulated at the stage of adding Antioxidant 6Cthereto, resulting in failure to obtain a rubber cement.

Results of determination and evaluation in the foregoing Examples 2 ˜3and Comparative Examples 3˜6 are shown in the following Table 2.

TABLE 2 Determination. Comp. Comp. Comp. Comp. evaluation items Ex. 2Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 3 Heat resistance Pre-heat aging 2H 2H H 2H2H 2H pencil hardness Post-heat aging 3H 5H 4H 5H 6H 3H pencil hardnessTackiness At press cross- none none found none none none linking Atantitack agent none none none none found none treatment

Example 4

To determine distribution of antioxidant concentration in Antioxidant6C-diffused rubber layer formed to a thickness of 150 μm in a metal-NBRlaminate material obtained in the same manner as in Example 3, therubber layer was sliced into thin pieces at every depth of 10 μm fromthe surface by a microtome, and the antioxidant in the sliced sampleswere individually extracted with a solvent, and the antioxidant in theindividual extracts was quantitatively determined by liquidchromatography method. The results are shown in the following Table 3,where the values in parentheses are parts by weight (pbw) in terms ofcharge basis.

TABLE 3 Depth from the surface (μm) Antioxidant concentration (wt. %) 0-10 2.34(3.9 pbw) 20-30 2.04(3.4 pbw) 40-50 1.75(2.9 pbw) 60-701.56(2.6 pbw) 70-80 1.39(2.3 pbw) 140-150 0.73(1.2 pbw)

INDUSTRIAL UTILITY

The following effects can be obtained according to the present process.

(1) The present process is based on post-addition of an amine-basedantioxidant to diffuse into the rubber layer up to the necessary amount,so such adverse effects as cross-linking inhibition by the antioxidantat the time of cross-linking, lowering of physical properties, etc. canbe made less to improve the heat resistance. Furthermore, much morediffusion of the antioxidant in the surface layer region moresusceptible to thermal damages leads to effective improvement of heataging resistance.

(2) The rubber material containing a large amount of the initially addedamine-based antioxidant suffers insufficient cross-linking, resulting inpoor material characteristics (see Comparative Example 4), and when thereaction time is prolonged, a portion of the antioxidant is consumed inthe cross-linking reaction, resulting in poor heat resistance (seeComparative Example 5), whereas the present process has effectivelysolved such a problem as the inhibition of initial charge of a largeamount of the antioxidant.

(3) Also in the case of forming the rubber layer from a rubber latex,addition of the antioxidant to the aqueous latex gives rise to polymercoagulation, so addition of the effective antioxidant is inhibitive,whereas the present process has successfully solved such a problem.

(4) Particularly, in the case of using an aqueous latex, the presentprocess has succeeded in improvement of moldability and heat resistanceat the same time as already explained before, and furthermore in thecase of using the antitack agent together with the antioxidant at thecoating stage, a further prominent effect is expectable. That is, thesimultaneous treatment with the antioxidant and the antitack agent cancontribute to an improvement of the heat resistance and surface tackprevention at the same time, resulting in cost reduction.

(5) In such a step of continuously molding and successively coiling asheet-form rubber product, it has been so far essential to form asurface protective layer mainly directed to surface tack prevention. Inthe present process, on the other hand, no special treating step isrequired for the tackiness prevention, thereby enabling such acontinuous molding without any product cost increase.

1. A process for producing an aging-resistant metal-rubber layerlaminate material having a rubber layer wherein the rubber layer has athickness of 5 to 300 μm and is formed from an aqueous latex, whichprocess comprises: applying a coating liquid containing an amine-basedantioxidant in solution or dispersion to a surface of a cross-linkedrubber layer composited with a metallic material, followed by heating toa temperature of 80° C. or higher, thereby diffusing the amine-basedantioxidant into the rubber layer.
 2. A process for producing anaging-resistant metal-rubber layer laminate material according to claim1, wherein the heating is carried out at 80° to 300° C.
 3. A process forproducing an aging-resistant metal-rubber layer laminate materialaccording to claim 1, wherein the amine-based antioxidant is diffusedinto the rubber layer in a proportion of 0.1 to 10 wt. % on the basis ofthe rubber material.
 4. A process for producing an aging-resistantmetal-rubber layer laminate material according to claim 1, wherein thecoating liquid contains an antitack agent together with the amine-basedantioxidant.
 5. A process for producing an aging-resistant metal-rubberlayer laminate according to claim 4, wherein the antitack agent is usedin a ratio by weight of 0.5 to 300 to the amine-based antioxidant.
 6. Aprocess for producing an aging-resistant metal-rubber layer laminatematerial according to claim 1, wherein the rubber layer is formed byperoxide cross-linking.